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Diamond planet 55 Cancri e may not be so precious after all

In the sky with diamonds? A so-called super-earth, planet 55 Cancri e was believed to be the first known planet to consist largely of diamond, due in part to the high carbon-to-oxygen ratio of its host star. Image credit: Haven Giguere/Yale University/Nasa

The smallest of several planets in the 55 Cancri system, the former 'diamond planet' is seen orbiting its host star at very close range in this artist's impression. A nearby brown dwarf with its own 'miniature planetary system is pictured as well. Image credit: Nasa/JPL-Caltech

A simulation of the silhouette of planet 55 Cancri e passing in front of ('transiting') its parent star, compared to the Earth and Jupiter transiting our Sun, as seen from outside the Solar System. Image: Jason Rowe/Nasa Ames and SETI Institute and Jaymie Matthews/UBC

A “super-Earth” planet 40 light years from our solar system, believed to be the first-ever discovered planet to consist largely of diamond, may not be quite as precious, according to new research led by University of Arizona astronomy graduate student Johanna Teske.

Revisiting public data from previous telescope observations, Teske’s team analysed the available data in more detail and concluded that carbon – the chemical element diamonds are made of – appears to be less abundant in relation to oxygen in the planet’s host star – and by extension, perhaps the planet – than was suggested by a study of the host star published in 2010.

“The 2010 paper found that ’55 Cancri’, a star that hosts five planets, has a carbon-to-oxygen ratio greater than one,” Teske said. “This observation helped motivate a paper last year about the innermost planet of the system, the ‘super-Earth’ 55 Cancri e. Using observations of the planet’s mass and radius to create models of its interior that assumed the same carbon-to-oxygen ratio of the star, the 2012 paper suggested the planet contains more carbon than oxygen.”

Observations obtained in 2010, together with simulations astronomers use to model a planet’s interior based on data like radius, mass and orbital velocity, had yielded a carbon to oxygen ratio greater than one, in other words, an alien world based on carbon instead of oxygen as most planets are in our solar system, including Earth.

“The Sun only has about half as much carbon as oxygen, so a star or a planet with a higher ratio between the two elements, particularly a planet with more carbon than oxygen, is interesting and different from what we have in our solar system,” explained Teske.

Based on the previous results, it was suggested that the “diamond planet” is a rocky world with a surface of graphite surrounding a thick layer of diamond instead of water and granite like Earth.

The new research by Teske and collaborators, to be published in the Astrophysical Journal and available online, calls this conclusion in question, making it less likely that a hypothetical space probe sent to sample the planet’s innards would dig up anything sparkling.

Teske’s group found that the planet’s host star contains almost 25 per cent more oxygen than carbon, about mid way between the Sun and what the previous study suggested.

“In theory, 55 Cancri e could still have a high carbon to oxygen ratio and be a diamond planet, but the host star does not have such a high ratio,” Teske said. “So in terms of the two building blocks of information used for the initial ‘diamond-planet’ proposal – the measurements of the exoplanet and the measurements of the star – the measurements of the star no longer verify that.”